The present invention relates to a printing process for the transfer of printing substance from an ink carrier onto an imprinting material, in which, with the help of an energy-emitting apparatus which emits energy during a process period in the form of electromagnetic waves, the printing substance undergoes a change in volume and/or position, and for example as a result a transfer of a printing point onto the imprinting material takes place, as well as a printing machine and a printing substance for this.
By a printing process is meant primarily a process for the reproduction as often as required of text and/or image patterns by means of a printing plate which is re-inked after each impression. In general, a distinction is made here between four basically different printing processes. Thus firstly the relief printing process is known, in which the printing elements of the printing plate are raised, while the non-printing parts are recessed. This includes for example letterpress printing and so-called flexographic or aniline printing. Furthermore, flatbed-printing processes are known in which the printing elements and the non-printing parts of the printing plate essentially lie in one level. These include offset printing in which strictly speaking the inked drawing on the printing plate is not printed directly onto the imprinting material, but is first transferred onto a rubber cylinder or a rubber blanket and only then is the imprinting material printed from this. Where in the following reference is made to imprinting material, however, this is to be understood as both the actual imprinting material, i.e. the material to be printed on, and any chosen transfer means, such as e.g. a rubber cylinder. A third process is the so-called gravure printing process in which the printing elements of the printing plate are recessed. A gravure printing process used industrially is so-called rotogravure printing. Finally, a porous printing process is also known in which at the printing positions the ink is transferred onto the imprinting material through screen-like openings of the printing plate.
These printing processes are all characterized by the fact that they require a printing plate which was more or less costly to produce, with the result that these printing processes operate profitably only with very long print runs, usually well over 1000 units.
For the printing of short print runs, printers are already used which are often connected to an electronic data processing system. These use generally digitally triggerable printing systems which are in a position to print individual printing points as required. Such printing systems use various processes with different printing substances on different imprinting materials. Some examples of digitally triggerable printing systems are: laser printers, thermal printers and ink jet printers. Digital printing processes are characterized by the fact that they do not require printing plates.
Thus for example an electro-thermal ink jet printing process is known for example from GB 2 007 162, in which the water-based ink is briefly heated in a suitable ink jet by electrical pulses until it boils, with the result that a gas bubble suddenly forms and an ink drop is ejected from the jet. This process is generally known by the term “bubble-jet”. However, these thermal ink printing processes have in turn the disadvantage that on the one hand they consume a great deal of energy for the printing of an individual printing point and on the other hand they are suitable only for printing processes which are water-based. Furthermore, every single printing point must be triggered separately by the jet. On the other hand, piezoelectric ink printing processes suffer from the disadvantage that the required jets are easily blocked, with the result that only special and expensive inks can be used for this.
It is known from DE 197 46 174 that a laser beam, through very short pulses in a printing substance which is located in cells of a printing roller, induces a procedure with the result that the printing substance undergoes a change in volume and/or position. As a result, the printing substance spreads over the surface of the printing plate and it is possible to transfer a printing point onto an imprinting material moved up against same. However, in this process it is disadvantageous that the filling of the cells is very difficult due to the small diameter of the cells. Therefore it is proposed in DE 100 51 850 to apply the printing substance essentially forming a continuous film to the ink carrier. The energy can be either transferred directly into the printing substance or firstly into an absorption layer which is applied to the ink carrier, and which in turn emits the energy to the printing substance. In the first case, special printing substances must be used which are capable of absorbing the energy. This severely restricts the variety of printing inks that can be used. In addition, the absorption of the light in the printing ink takes place within a relatively large volume passed through by the laser beam. With some inks the energy is also not completely absorbed. The absorption is also strongly dependent on the printing substance used and the actual thickness of the printing substance on the ink carrier. Due to the relatively large volume in which the energy is absorbed, a relatively large amount of energy must be introduced into the printing substance in order to induce the change in volume and/or position of the printing substance necessary to set a printing point. Moreover, a delay in boiling often occurs with the result that the temperature at which gas bubbles form in the printing substance cannot be predicted. This means that the absorption—and the local heating of the printing substance associated therewith—takes place largely uncontrolled, which results i.a. in a marked variation in printing-point size. To ensure that the desired printing point is set in each case, much more energy must therefore be introduced into the printing substance than is usually necessary to induce the desired change in position and/or volume of the printing substance.
If on the other hand the printing substance is heated indirectly by introducing the energy firstly into an absorption layer, the required energy is generally even higher as, in addition to the printing substance, the absorption layer must also now be heated. In addition, the heat conduction in the absorption layer must not be ignored, with the result that a larger area of the absorption layer is necessarily heated, which likewise increases the required energy. Moreover, with the known processes, even a detachment of the absorption layer often occurs.